In the lumbar lateral motor column of the chick embryo, 40% of the cells die between 6 1/2 and 9 1/2 days of incubation, reducing the population from 20.000 to approximately 12.500. The time of cell death coincides with the nerve ingrowth into the leg, suggesting that the cells which die loose out in a competition of their axons at the periphery. The terminal mitoses (birthdates) of lateral motor neuroblasts in the brachial and lumbar columns have been identified by autoradiography of early stages. They occur between 51 and 64 hours of incubation in the brachial column and around 72 hours in the lumbar column. Using electromyography on synergistic and antagonistic leg muscles, antagonistic inhibition was found as early as at 7 days of incubation, that is, shortly after onset of leg mortility and before the proprioceptive feedback is established. The EMG patterns were described for representative stages between 7 days and hatching, and characterized qualitatively and quantitatively.